1. Field of the Invention
The present invention relates to an .alpha.-aluminium oxide (hereinafter referred to as ".alpha.-Al.sub.2 O.sub.3 ") based ceramic composite which exhibits superior wear-resistance and improved toughness and; almost free from breakage or chipping of a cutting edge not only when continuously cutting steel or cast iron, but also when discontinuously cutting steel or cast iron, which requires a high toughness, together with excellent cutting performance.
2. Description of the Related Art
The conventionally known ceramic materials for this purpose include Al.sub.2 O.sub.3 -ZrO.sub.2 ceramics and Al.sub.2 O.sub.3 -ZrO.sub.2 -MgO, Y.sub.2 O.sub.3 and CeO.sub.2 ceramics disclosed in Japanese Patent Publication No. 59-6,274, which have microstructures an example of which is shown in the structural photograph based on a scanning-type electron microscope given in FIG. 1. These ceramics are used for cutting tools mainly for continuous cutting of steel.
Conventional Al.sub.2 O.sub.3 -ZrO.sub.2 ceramics cannot fully satisfy these requirements because of the easy occurrence of chipping in discontinuous cutting of steel, for example.
As disclosed in Japanese Patent Provisional Publication No. 61-53,154, on the other hand, there are also known Al.sub.2 O.sub.3 -ZrO.sub.2 -MgO, Y.sub.2 O.sub.3, CeO.sub.2 -TiC, TiN, TiCN and SiC ceramics, which tend however to easily chip when used for discontinuous cutting of steel.
In addition, Japanese Patent Provisional Publication No. 2-283,661 discloses .alpha.-Al.sub.2 O.sub.3 -CeO.sub.2 ceramics used as grinding grains for polishing.
Furthermore, Japanese Patent Provisional Publication No. 4-238,861 discloses ZrO.sub.2 -.alpha.Al.sub.2 O.sub.3 -.beta.-Al.sub.2 O.sub.3 ceramics. However, .beta.-Al.sub.2 O.sub.3 which takes the form of acicular grains, cannot be used as a matrix component material because of the content of from 3 to 10 vol. %.
A paper appearing in J. Am. Ceram. Soc., 73 [7] 2077-85 (1990) reports that addition of combinations of Na.sub.2 O and SiO.sub.2, CaO and SiO.sub.2 SrO and SiO.sub.2 or BaO and SiO.sub.2 to Al.sub.2 O.sub.3 powder and sintering of the mixture causes abnormal grain growth of .alpha.-Al.sub.2 O.sub.3 grains. The grains having abnormally grown have a plate-like shape with a particle size of over 100 .mu.m and an aspect ratio of at least 5, according to the report.